Resonant directional coupler with square guide



P. G. MARIE July 28, 1959 RESONANT DIRECTIONAL COUPLER WITH SQUARE GUIDEFiled July 2, 1956 INVENTOR PIERRE G. MARIE By W -vc, W04 -w4 UnitedStates Patent Office RESONANT DIRECTIONAL COUPLER WITH SQUARE GUIDEPierre G. Mari, Paris, France Application July 2, 1956, Serial No.595,460 Claims priority, application France July 4, 1955 2 Claims. (Cl.333-) The present invention relates to resonant directional couplerscomprising two rectangular guides which are connected together by acircular guide stub forming a cavity resonator coupled to the formerguides by systerns of slots and, more particularly, to resonantdirectional couplers of this type in which one of the rectangular guidesis square.

In the specifications of U.S. applications Ser. No. 416,869 and 593,886,respectively, filed March 17, 1954, and June 26, 1956, in the name ofthe present applicant, for resonant directional couplers, directionalcouplers are described which comprise two identical rectangularwave-guides and a circular cavity resonator which connects themtogether, such that a traveling wave, which progresses in one of therectangular guides in the TB mode and arrives in the region of junctionbetween the said rectangular guide and the cavity resonator, has itselectrical energy split into two parts. The first part (that which hastrequencies outside the frequency band of the cavity resonator)continues to be propagated in the rectangular guide to beyond thejunction with the cavity resonator. The second part of this energy (thatwhich has frequencies within the frequency band of the cavity resonator)is radiated in the circular resonator through the intermediary of asystem of special coupling slots in the form of a circularly polarizedwave, which is constituted by two waves of the TB mode which arepolarized perpendicularly and in quadrature. This circularly polarizedwave, in turn, radiates in the second rectangular guide a wave of the TEmode, the

direction of propagation of which depends upon the direction ofpropagation of the incident wave in the first rectangular guide.

The system of coupling slots may be of two types. In the couplers ofapplication Ser. No. 416,869, the slot systems comprise two crossedconcentric slots, the centre of which is situated on the axis of thecircular guide but is eccentric in relation to the axis of eachrectangular guide. The result of this is that the structure of thecoupler is asymmetric, the axis of the circular guide not meeting theaxes of the rectangular guides. In the couplers of. application Ser. No.593,886, the slot systems comprise two separate slots, one of which islongitudinal and the other is transverse in relation to the rectangularguide in which they have been made, and which are situated on one sideand the other of the axis of the latter guide. The distances of theslots to the axis should have well defined values which are explained inthe specification of the aforesaid application. In this case, thestructure of the coupler is symmetrical in the sense that the axis ofthe circular guide meets the axes of the rectangular guides.

In both cases, the slots, which are supposed to be seen from theinterior of one of the rectangular guides, have an asymmetry in relationto the axis of the guide either by both of them being on the same sideor by being on one side and the other but with difierent directions oforientation and with unequal distances to the I 2,897,457 Patented July28, 1959 axis. It follows from this that the small side of therectangular guides should have a dimension which is less than (where Xis the wave length in unbounded space of the wave which passes throughthe coupler) in order that, at the coupling position, the wave of the TBmode, which is propagated in the rectangular guide, should not giverise, at least partially, to a wave of the TE mode which has, in therectangular guide, a direction of polarization which is perpendicular tothe direction of polarization of the wave of the 'I'E mode.

Now, in certain applications, the couplers should have one of theirrectangular guides square in order to be able to convey two TE waveswhich have their directions of polarization perpendicular. This is thecase, for example, where a plurality of couplers of the aforesaid kindhave a common square guide and form a multi-channel filter, all thechannels or some of them being duplicated and used simultaneously fortransmission and for reception. The electromagnetic energy relating tothe transmitting channel arrives through a rectangular guide and iscoupled, through the intermediary of a first directional coupler, withthe square guide which leads it to an aerial with a certain direction ofpolarization, in accordance with the TB mode in said square guide. Theelectromagnetic energy relating to the receiving channel (assumed tohave the same bandwith than the transmitting channel) arrives from theaerial through the square guide with a direction of polarizationperpendicular to the preceding, in accordance with the TE mode in saidsquare guide, and it is coupled with a rectangular guide of a seconddirectional coupler. It appears that, in order to obviate any mixture ofthe energies of the channels, it is necessary that, at the junctionpositions between the square guide and the circular cavity resonators orguides of the directional couplers, no wave of the TE mode should becreated by the wave of the TE mode and vice versa.

Furthermore, in the couplers of the type described disclosed in theprior art, the coupling means are adapted to pick up from therectangular guide two phase quadrature field components and to applythem to the circular cavity resonator in appropriate space relationship.The first field component to be picked up is either the transverseelectric field component or the transverse magnetic field componentwhich are cophasal. The second field component is the longitudinalmagnetic field component, in phase quadrature with the two first. Now itis known that said last component has a phase which vary in thelongitudinal direction of the rectangular guide and it results therefromthat the true phase quadrature relationship only occurs in 'a transverseplane. As the longitudinal magnetic field component is coupled through alongitudinal slot, the field fringing through the slot is not cophasalthroughout said slot, which results in poor coupling versus frequencyresponse.

The object of the invention is to construct a slot-andprobe couplingsystem between a rectangular guide (which may be square) and a circularguide, which is symmetrical in relation to the axis of the rectangularguide and is such that a wave of the TB mode in the rectangular guidegives rise to a circularly polarized wave in the circular guide.

Another object of the invention is to provide a slot and probe couplingsystem between a rectangular or square 3 by 90 and for applying them inparallel space relation ship to the circular guide.

Another object of the invention is to construct a multichannel filtercomprising a plurality of directional couplers having a common squareguide which is traversed by the energies of two waves havingperpendicular directions of polarization in this square guide and thesame band or hands overlapping with each other, relating to twocommunication channels, which ,enables the said channels to beseparated.

The directional coupler of the invention comprises, essentially, arectangular guide, a circular guide coupled to the former guide by atwo-slot system, a square guide and means, for coupling the square guideand the circular guide, composed of a slot which is diametral withrespect to the circular guide, transverse with respect to the squareguide and symmetrical with respect to the longitudinal axis ofthelatter, a probe which is axial with respect to g ofthe magnetic field aa first m, wave in circular guide the circular guide, fixed to thelatter by a radial conductive fixing rod and passes into the squareguide at the center of the slot, and,'a tuning plug inside the circularguide and opposite the'end of the radial fixing rod, and a tuning pluginside the square guide and opposite the end of the probe.

The branching filter which may be made with couplers according to theinvention comprises a plurality of directional couplers of the abovementioned type, the square guides of which are connected together andaligned with one anotherand the circular guides of which are connectedeither to one side of the square guide or to another side of the squareguide which is perpendicular to the former side, according as to whetherthe said couplers are intended to extract from the total wave, which ispropagated in the square guide, partial waves having a certain directionof polarization or the perpendicular direction of polarization.

The invention will now be described in detail with reference to theaccompanying drawings, of which Fig. 1 represents a directional coupleraccording to the invention, and I Fig. 2 represents a multi-channelfilter constituted by couplers of the type shown in Fig. 1.

Referring to Fig. 1, 1 is a square guide in which two traveling waveshaving directions of polarization indicated by the arrows 3 and 4 arepropagated simultaneously. 9 is a circular cavity resonator or guidestub which is coupled with the guide 1 by a coupling system according tothe invention and coupled with a rectangular guide 2 by a slot systemcomprising the slots 22 and 23 of the type described in thespecification of US. application Ser. No. 593,886. Naturally, the systemof coupling slots between the guides 8 and 9 could be of any'prior-arttype, of the type of US. application 416,869 for example.

The coupling between the guide 1 and the circular guide 9 is effected bya single rectangular slot 5 which is terminated by two rounded portionsand the longitudinal axis of which is in the cross-section plane of thesquare guide 1.

At the center of this slot and perpendicularly to this plane there isarranged a cylindrical rod 6 which constitutes a probe, the axis ofwhich coincides with that of the circular guide 9. One of the endsof'the rod 6 is opposite a tuning plug 7 placed inside the squareguide 1. The other end of the probe 6 is kept fixed to the wall of thecircular guide 9 by a fixing conductive rod 8.

The axis of the rod 8 is perpendicular to the axis of the guide 9 and,consequently, to the axis of the probe 6. Situated in the extension ofthe axis of the rod 8 is a cylindrical rod 10 provided with ascrew-thread. This rod is screwed into the guide 9 and its end, which issituated outside the guide, is provided with a milled head 11 whichenables the said rod to be turned in order to make it pass, to a greateror smaller extent, into the circular guide 9.

The slot 5 couples the transverse magnetic field parallel to arrow 4 insquare guide 1 to the transverse component 9, said component'beingparallel to the slot. The probe 6 and the rod 8 couple the transverseelectric field parallel to arrow 3 in square guide 1 to the u'ansverseelectric field of a second TE wave in circular guide 9, said lasttransverse electric field being parallel to the rod 8 and consequentlyto the slot 5. There are thus produced in the guide 9 twoelectromagnetic fields having rectangular patterns but which would becophasal if no precautions were taken as the transverse magnetic fieldand the electric field in guide 1 from which they derive are cophasal.

In order to put these two fields into quadrature relationship, the screw10 is turned so as to sharply detune the cavity as regards the secondwave having its electric field parallel to the rod 8. Thus the phase ofthe vibration of said wave may be phase shifted by from the drivingelectric field phase in guide 1 (see for example "Principles ofMicrowave Circuits," by C. G. Montgomery et al., McGraw-Hill BookCompany Inc., New York, 1948, page 226). The screw 10 is of littleeffect upon the first wave which has an electric field perpendicular tothe screw and besides because it is located near the center of thecylindrical resonator and, as the resonator vibrates in the TB mode, thetransverse magnetic field of the wave is almost zero at this point. Thetwo fields having rectangular patterns and being apart in phase giverise to a TE mode circularly-polarized standing wave. It might be shownby a similar argument that such a standing wave produces in therectangular 1 a traveling wave which progresses towards one end or theother according to the sense of rotation of said standing wave. Screw 7serves to cancel the admittance due to the post 6, partially crossingthe guide 1.

Referring now to Fig. 2, 12 is a square general guide, which isconnected, at one end, to a pyramidal horn 13 and is terminated, at theother end, by a dissipative load 14. The pyramidal born 13 serves, atthe same time, as a transmission aerial for a first communicationchannel with a direction of polarization parallel to arrow 3, and as areception aerial for a second communication channel with a direction ofpolarization parallel to arrow 4. The dissipative load 14 is constitutedby two small rectangular absorbent plates. On the side 15 of the squareguide 12 is connected a directional coupler 17, the rectangular guide ofwhich is terminated, at one end, by a dissipative load 18 and isconnected, by its other end 19, to a transmitter (not shown) whichproduces the electromagnetic energy relating to the first channel.

Connected on the side 16 of the square guide 12 is a directional coupler20, the rectangular guide of which is terminated, at one end, on adissipative load 21 and is connected, by its other end 24, to a receiver(not shown) which receives the electromagnetic energy relating to thesecond channel.

Naturally, the number of directional couplers, which form the branchingfilter of Fig. 2, may be more than two.

What I claim is:

1. A directional coupler comprising a wave guide of squarecross-section, a circular cavity resonator, a rectangular guide coupledto said circular cavity resonator by a two-slot system, coupling acircularly polarized TE mode standing wave in the circular cavityresonator to a TE mode traveling wave in the rectangular guide, and acoupling system between said square guide and said circular cavityresonator comprising a slot transverse with respect to the axis of thesquare guide and diametrical with respect to the circular cavityresonator coupling the trans verse magnetic field of a TE mode wave inthe square probe, radial with respect to the circular cavity resonatorand parallel to said slot, said probe and rod coupling the transverseelectric field of the TB mode wave in the square guide and thetransverse electric field of a second standing wave of the TB mode insaid resonator and a tuning plug inside the circular cavity resonatorand opposite the radial metallic rod, whereby said plug and rod detunethe cavity resonator for the second standing wave, allowing said wave tobe in quadrature relationship with the first standing wave.

2. A directional coupler comprising a general square guide conveying aTE mode wave and a TE mode wave, a first circular "cavity resonatorcoupled to a first face of said square guide by a first coupling systemcomprising a slot perpendicular to the general square guide axisanddiametrical with respect to the circular cavity resonator, a probecoaxial with said resonator and entering through the slot inside thesquare guide, a metallic rod supporting the probe, radial with respectto the first cavity resonator and parallel to the slot of said firstcoupling system, and a tuning plug inside said first resonator andopposite the rod, afirst rectangular guide coupled to said firstcircular cavity resonator by a two-slot coupling system, a secondcircular cavity resonator coupled to a second face of the square guideperpendicular with the first face by a second coupling system identicalwith the first coupling system and a second rectangular. guide coupledto said second circular cavity resonator by a secondtwo-slot couplingsystem, whereby the energy of the TB mode wave is collected at one endof the first rectangular guide and the energy of the TB, mode wave iscollected at one end of the second rectangular guide.

, References Cited in the file of this patent UNITED STATES PATENTS

